The present invention relates to circuits for driving power transistors which control the application of electricity to a load; and more particularly to such circuits which provide some degree of protection against the adverse effects of a short circuit in the load coupled to the transistor.
A recent addition to the family of power semiconductor devices are insulated gate bipolar transistors (IGBT). This type of device is adapted for use in power supplies and other applications where it is required to switch currents on the order of several hundred amperes. One application of IGBT's is in high frequency inverters of X-ray generators.
A desirable feature of this type of power semiconductor device, compared to thyristors, is the capability of surviving short circuit load conditions by self-limiting the fault current rather than relying solely upon conventional protection techniques. This self-limiting capability is a function of the conductivity of the IGBT and the magnitude of the drive voltage applied to its gate electrode. Higher gat voltages permit a greater fault current to flow through the transistor; thereby increasing the stress on the device and likelihood that it will fail under a short circuit condition before a current sensor can act to turn off the transistor's gate drive. It is therefore advantageous from the aspect of short circuit survival to limit the conductivity of the transistor, but this has the adverse effect of raising the on-state voltage drop across the IGBT. A higher voltage drop results in a larger power loss in the device and more power dissipation. When the IGBT is switching several hundred amperes, a difference of a few volts across the device amounts to a significant power dissipation.
As a consequence, a designer seeking to incorporate an IGBT into a power switching circuit has been faced with the dilemma of choosing between a relatively high gate drive voltage in order to reduce the power dissipation in the device, while reducing short circuit protection; or utilizing a lower gate drive voltage to increase the short circuit survivability, while increasing the power dissipation of the device.